This invention relates to phototransistors.
More particularly, the present invention relates to two terminal edge illuminated heterojunction bipolar phototransistors (HBPTs).
As the bit rates of telecommunication and data communication systems increase, the demands on the performance requirements of photoreceivers increase. Photoreceivers are used to convert incident light pulses into electrical current. As bit rates extend beyond 40 Gbit/s, the sensitivity of optical receivers tends to decrease, causing degradation in the overall performance of the optical communications link. Receiver sensitivity has been improved in the prior art by implementing either avalanche photodetectors (hereinafter referred to as xe2x80x9cAPDsxe2x80x9d) or heterojunction bipolar phototransistors (hereinafter referred to as xe2x80x9cHBPTsxe2x80x9d) as the optical detection element.
APDs improve the receiver sensitivity by providing internal optical to electrical gain through the avalanche multiplication process. Some of the problems associated with implementing APDs in the receiver circuits are that the avalanche multiplication process is an inherently noisy process and requires excessively high bias voltages on the order of 40 volts to achieve the desired gain. The high electric fields that result from these excessively high bias voltages lead to reliability problems that cause premature failure. Many engineering solutions need to be implemented to circumvent these issues. As such, the fabrication and device layer profile are highly specialized for the APD, which prevents the monolithic integration of the APD with the transimpedance amplifier (TIA) circuit. The resulting consequence of this specialization is that it is unlikely that front-end optical receivers that are based on APDs will be able to operate at 40 Gbps bit rates or beyond due to the excessive parasitic losses that come from the hybrid integration of the APD with the rest of the circuit.
An improvement over the APD is the HBPT. Typically, the HBPT is designed in a manner similar to heterojunction bipolar transistors (HBTs) that are optimized for electrical circuit performance where the doping and the thickness of the base layer are chosen to minimize the lateral base resistance. In typical HBT devices, the base is doped around 1xc3x971020 cmxe2x88x923 and the base thickness is around 500 xc3x85. The lateral base resistance is an important parameter in the design of HBT circuits as it adversely affects the fmax, which is a FIGURE of merit that is known to those familiar with the art. Unfortunately, the high doping levels in the base and the thickness of the base reduce the current gain and bandwidth of the HBT because of excess recombination rates due to Auger recombination and the increased base transit times. In the prior art, a high base doping and large base layer thickness are deemed necessary to reduce the lateral base resistance to optimize the overall performance of HBTS. What is desired at high bit rates is a device structure that can improve the gain-bandwidth product of the HBPT. The gain-bandwidth product can be significantly improved by making a modification to the standard HBT epilayer profile.
When the HBT is used as an optical detector (HBPT), the base resistance is less important since the HBT is controlled by an optical signal rather than an electrical signal applied to its base terminal. Hence, minimizing the lateral base resistance in the HBPT has less benefits and the gain-bandwidth product of the HBPT is unnecessarily sacrificed. As such, the base doping and base layer thickness can be significantly reduced in order to increase the gain-bandwidth product of the HBPT device.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly, it is an object of the present invention to provide a new and improved two terminal edge illuminated heterojunction bipolar phototransistor.
It is an object of the present invention to provide a new and improved two terminal edge illuminated heterojunction bipolar phototransistor which has a superior gain-bandwidth product compared to current phototransistor devices.
It is another object of the present invention to provide a new and improved two terminal edge illuminated heterojunction bipolar phototransistor which has a short carrier transit-time.
And another object of the invention is to provide a new and improved two terminal edge illuminated heterojunction bipolar phototransistor which has a high internal quantum efficiency.
Still another object of the present invention is to provide a new and improved two terminal edge illuminated heterojunction bipolar phototransistor which has a high external coupling efficiency.
A further object of the invention is to provide a new and improved two terminal edge illuminated heterojunction bipolar phototransistor which has the ability to perform at bit rates greater than 40 Gbits/second.
To achieve the objects and advantages specified above and others, a two terminal edge illuminated epilayer waveguide phototransistor (hereinafter referred to as xe2x80x9cWPTxe2x80x9d) is disclosed which includes a subcollector layer formed from an epitaxially grown quaternary semiconductor material that is grown on a semiconductor substrate. The epitaxially grown quaternary semiconductor material improves the optical waveguide mode properties. A collector region is epitaxially grown on the subcollector layer. A base region is epitaxially grown on the collector layer. An emitter region is then epitaxially grown on the base region. The various layers and regions are formed so as to define an edge-illuminated facet for receiving incident light. Further, ohmic contacts are formed to the subcollector and emitter regions to allow electrical signals to be extracted from the phototransistor. Finally, the base does not have an ohmic contact so that the base thickness can be minimized.
In a preferred embodiment, the subcollector region consists of an InGaAsP quaternary semiconductor with a composition that corresponds to a bandgap wavelength of 1.15 xcexcm. The InGaAsP subcollector is a unique advantage that allows the optimization of the input optical coupling efficiency without sacrificing the phototransistor""s electrical performance. The InGaAsP subcollector expands the optical mode in the vertical direction, which increases the input mode coupling efficiency to commercially available lensed optical fibers without degrading the electrical properties of the device. The heavily doped InGaAsP subcollector also maintains the necessary electrical characteristics needed for high performance device operation. In addition, the base region does not contain an ohmic base contact layer so that the base region thickness can be minimized and, consequently, the current gain is increased. The base region is also moderately doped. By removing the ohmic base contact layer and minimizing the base doping, the optical to electrical conversion gain is increased and the base transit time is reduced. This improvement in performance will allow the WPT discussed here to perform at higher bit rates.
The waveguide phototransistor discussed here will eliminate all of the previously mentioned issues associated with the APD and improve the performance of phototransistors typically used as photoreceivers. The waveguide phototransistor uses a subcollector region that expands the optical mode size vertically without degrading the electrical properties of the device. Expanding the optical mode size in this manner increases the input optical coupling efficiency. Also, the WPT has improved performance because the doping and the thickness of the base layer are chosen to maximize the gain-bandwidth product (fT).
The waveguide phototransistor geometry has inherent advantages over top-illuminated phototransistors that have been demonstrated in the prior art. Some problems associated with the top-illuminated approach include the fact that the thickness of the absorbing layers must be increased to above 1 xcexcm in order to absorb greater than 90% of the incident light. This leads to poor frequency response of the top-illuminated phototransistor due to the excessive base and collector carrier transit-times. The waveguide phototransistor geometry solves this problem because the light propagates and gets absorbed down the length of the device in a direction that is orthogonal to the flow of electrical carriers. As such, the thickness of the absorbing layers can be kept small such that the base and collector transit-times are short which allows for high-speed operation.